The modem communications industry began with the development of gridded vacuum tube amplifiers. Microwave vacuum tube devices, such as power amplifiers, are essential components of microwave systems including telecommunications, radar, electronic warfare and navigation systems. While semiconductor microwave amplifiers are available, they lack the power capabilities required by most microwave systems. Vacuum tube amplifiers, in contrast, can provide microwave power which is higher by orders of magnitude. The higher power levels are because electrons can travel faster in vacuum with fewer collisions than in semiconductor material. The higher speeds permit larger structures with the same transit time which, in turn, produce greater power output.
In a typical microwave tube device, an input signal interacts with a beam of electrons. The output signal is derived from the thus-modulated beam. See, e.g., A. S. Gilmour, Jr., Microwave Tubes, Artech House, 1986, 191-313. Microwave tube devices include triodes, tetrodes, pentodes, klystrodes, klystrons, traveling wave tubes, crossed-field amplifiers and gyrotrons. All contain a cathode structure including a source of electrons for the beam, an interaction structure (grid or gate), and an output structure (anode). The grid is used to induce or modulate the beam.
Conventional vacuum tube devices are typically fabricated by mechanical assembly of the individual components. The components are made separately and then they are secured on a supporting structure. Unfortunately, such assembly is not efficient or cost-effective, and it inevitably introduces misalignment and asymmetry into the device. Attempts to address these problems have led to use of sacrificial layers in a rigid structure, i.e., a structure is rigidly built with layers or regions that are removed in order to expose or free the components of the device. See, e.g., U.S. Pat. No. 5,637,539 and I. Brodie and C. Spindt, “Vacuum microelectronics,” Advances in Electronics and Electron Physics, Vol. 83 (1992). These rigid structures present improvements, but still encounter formidable fabrication problems.
The usual source of beam electrons is a thermionic emission cathode. The emission cathode is typically formed from tungsten that is either coated with barium or barium oxide, or mixed with thorium oxide. Thermionic emission cathodes must be heated to temperatures around 1000 degrees C. to produce sufficient thermionic electron emission current, e.g., on the order of amperes per square centimeter. The necessity of heating thermionic cathodes to such high temperatures creates several problems. The heating limits the lifetime of the cathodes, introduces warm-up delays, requires bulky auxiliary equipment for cooling, and interferes with high-speed modulation of emission in gridded tubes.
While transistors have been miniaturized to micron scale dimensions, vacuum tubes have been much more difficult to miniaturize. This difficulty arises in part because the conventional approach to fabricating vacuum tubes becomes increasingly difficult as component size is reduced. The difficulties are further aggravated because the high temperature thermionic emission cathodes used with conventional vacuum tubes present increasingly serious heat and reliability problems in miniaturized tubes.
A promising new approach to microminiaturizing vacuum tubes is the use of surface micromachining to make microscale triode arrays using cold cathode emitters such as carbon nanotubes. See Bower et al., Applied Physics Letters, Vol. 80, p. 3820 (May 20, 2002). This approach forms tiny hinged cathode, grid and anode structures on a substrate surface and then manually releases them from the surface to lock into proper positions for a triode.
FIGS. 1A and 1B illustrate the formation of a triode microtube using this approach. FIG. 1(a) shows the microtube components formed on a substrate 1 before release. The components include surface precursors for a cathode 2, a gate 3 and an anode 4, all releasably hinged to the substrate 1. The cathode 2 can comprise carbon nanotube emitters 5 grown on a region of polysilicon. The gate 3 can be a region of polysilicon provided with apertures 6, and the anode 4 can be a third region of polysilicon. The polysilicon regions can be lithographically patterned in a polysilicon film disposed on a silicon substrate. The carbon nanotubes can be grown from patterned catalyst islands in accordance with techniques well known in the art. The high aspect ratio of the nanotubes (>1000) and their small tip radii of curvature (˜1 to 30 nm), coupled with their high mechanical strength and chemical stability, make them particularly attractive as electron emitters. FIG. 1B shows the components after the release step, which is typically manually assisted. Release aligns the gate 3 between the cathode 2 and the anode 4 in triode configuration.
FIG. 2, which is useful in illustrating a problem to which the present invention is directed, is a scanning electron microphoto which shows an exemplary surface micromachined triode device. On the surface of the device substrate 10, e.g., a silicon nitride surface on a silicon wafer, are formed a cathode electrode 12 attached to the device substrate 10 by a hinge mechanism 13 and a spring 11. A grid 14 is attached to the device substrate 10 by a hinge mechanism 15, and an anode 16 is attached to the device substrate 10 by a hinge mechanism 17. Also on the substrate 10 are contacts 18 electrically connected to the cathode electrode 12, grid 14, and anode 16. The contacts 18 and connective wiring are typically polysilicon coated with gold, although other materials are possible. Design of the connective wiring should take into account the subsequent rotation of the cathode electrode 12, grid 14, and anode 16, to avoid breakage and/or reliability problems. The substrate 10 also has three locking mechanisms 24, 26, 28, which secure the cathode 12, grid 14, and anode 16 in an upright position, as discussed below. All these components, including the hinges, are formed by surface micromachining. The inset is a magnified view of the aligned and patterned carbon nanotubes (deposited on cold cathode), placed against the MEMS gate electrode (grid) 14 with corresponding openings.
The cathode electrode 12, with attached emitters 19, the grid 14, and the anode 16, are surface micromechanical and then mechanically rotated on their hinges, 13, 15, 17 and brought to an upright position substantially perpendicular to the surface of the device substrate 10. The locking mechanisms 24, 26, 28 are then rotated on their hinges to secure the cathode electrode 12, grid 14, and anode 16 in these upright positions. Vacuum sealing and packaging of the structure are then effected by conventional techniques.
In operation, a weak microwave signal to be amplified is applied between the grid and the cathode. The signal applied to the grid controls the number of electrons drawn from the cathode. During the positive half of the microwave cycle, more electrons are drawn. During the negative half, fewer electrons are drawn. This modulated beam of electrons passes through the grid and goes to the anode. A small voltage on the grid controls a large amount of current. As this current passes through an external load, it produces a large voltage, and the gridded tube thereby provides gain. Because the spacing between the grid and the cathode can be very small, a microtube (or other gridded tube) can potentially operate at very high frequencies on the order of 1 GHz or more.
The advantage of the surface micromachining is that little additional mechanical assembly is needed to construct a three dimensional structure. However, in order to achieve mechanical release and to maintain the three dimensional configuration achieved, the surface micromachined MEMS devices need mechanical parts such as flaps, support plates, notches, and hinges which take up significant real estate on the device surface.
While microtube device function has been demonstrated, the field emission efficiency needs further improvements. The intensity and performance of electron field emission are strongly dependent on the electric field applied between the cathode and the gate (grid) and the field between the cathode and the anode. The cathode-gate gap spacing needs to be controlled to a few micrometers. The manual flip-up of the micromachined electrodes into the desired vertical position fails to provide consistent control of the cathode-gate gap spacing, especially if there are inhomogeneities in the height of the nanotube emitters. Accordingly there is a need for an improved method of making vacuum microtube devices having more efficient use of substrate area and more precisely controlled electrode spacing.